1. Field of the Invention
This invention relates to an electrically conductive laminate and production thereof. More particularly, the present invention relates to a transparent electroconductive laminate composed of a transparent electroconductive layer which is composed mainly of an indium oxide formed on a planar substrate.
2. The Prior Art
With the advent of the "informationalized society," there has been great progress in producing a device which utilizes both properties of optics and electronics, i.e., an optoelectronics device. According to the great popularization of the microcomputer, the innovations of instruments surrounding these microcomputers are extensively advanced. A transparent touch panel is widely used as an input unit, and a liquid crystal display and an electroluminescent display are becoming widely used as an output unit.
In the above-mentioned transparent touch panel, the liquid crystal display and the electroluminescent display, a transparent electrode is used and, in particular, a transparent electroconductive laminate, especially a transparent electroconductive plastic laminate, is used. A transparent electroconductive layer in the electroconductive laminate used for this purpose requires particularly excellent durability and stability of the electrical properties against mechanical stresses.
For the transparent electroconductive layer, a thin layer of a metal such as gold (Au), palladium (Pd), etc., a thin layer of a metal oxide such as indium-tin oxide (ITO), cadmium tin oxide (CTO), stannic oxide (SnO.sub.2), titanium dioxide (TiO.sub.2), etc. and a multi-layer thin film such as titanium oxide (TiOx)/silver (Ag)/titanium oxide (TiOx), etc. are well known. Among these films, a thin layer of a metal oxide is superior to others in principal properties such as transparency, electroconductivity, mechanical properties, etc. In the thin layers of a metal oxide, a layer mainly composed of an indium oxide is advantageous, and of those an indium tin oxide (ITO) layer is especially excellent in transparency and electroconductivity. This further attracts attention since it can be readily etched to a desired electrode pattern (superior etchability).
Various production methods for a transparent electroconductive film composed mainly of indium oxide have been proposed.
The conventional process for the production of the glass-based film comprises the spraying of a solution of tin tetrachloride in hydrochloric acid onto a glass sheet heated to several hundred degrees Celsius, followed by oxidation at high temperature to form a thin film of stannic oxide. A recently developed method uses indium oxide as an evaporation source which is deposited as a vapor on a glass sheet (heated normally to about 300.degree. to 350.degree. C.) in a high degree of vacuum in the range of from 10.sup.-4 to 10.sup.-5 mmHg. Obviously, either method requires the heating of the glass base to high temperatures, as does another known method which supplies the vacuum system with water vapor or a gas that contains water vapor. The last mentioned method aims at forming an electrically conductive film of low resistivity, for example, 100 ohms/cm.sup.2 or less, but even that method requires heating of the glass base to a temperature between 300.degree. and 350.degree. C. In short, the heating of the glass base is indispensable to the manufacture of a desired glass-based electrically conductive film in the conventional technique, and apparently, such a technique is not applicable to the production of a thin electrically conductive film using a plastic base. The thus obtained indium oxide-based transparent electroconductive layer was a completely crystallized layer.
Several methods have been proposed to solve the stated problem, and most of them rely on a vacuum vapor deposition process using either indium oxide or indium as a primary evaporation source.
Examples of a method using indium oxide as an evaporation source are described in Japanese Patent Publication Nos. 35431/76 and 37667/76 wherein an indium oxide vapor is deposited on a plastic base in a high degree of vacuum (i.e., less than 1.times.10.sup.-3 mmHg, typically between 1.times.10.sup.-4 to 1.times.10.sup.-5 mmHg) that was either unheated or heated to a suitable temperature tolerable to the plastic base, followed by oxidative treatment under heating primarily in an oxidizing gas atmosphere. However, this method demands relatively severe conditions for the oxidation treatment that follows the vacuum deposition of indium oxide vapor; for example, the optimum temperature for heat treatment in air for a practical period is between 200.degree. and 250.degree. C. or higher. During vacuum deposition, indium oxide is decomposed to lower oxides, so the purpose of oxidative treatment is to convert them into higher oxides, but since the conditions for that treatment are severe, as mentioned above, only limited types of plastics can be used as the base. In addition, the indium oxide film thus prepared was also a completely crystallized layer.
The other method, that uses indium as an evaporation source, deposits the vapor of indium oxide on a plastic base that is formed by oxidizing indium in a vacuum system having a relatively low degree of vacuum (i.e., ca. 2.times.10.sup.-2 to 1.times.10.sup.-4 mmHg) which is being supplied with an oxidizing gas.
Japanese Patent Publication No. 14304/65 describes a method that facilitates the conversion of indium to indium oxide by heating the plastic base to at least 100.degree. C., and typically between about 110.degree. and 150.degree. C., before vacuum deposition. Japanese Patent Publication No. 8137/68 describes a method wherein a thinner film of vapor deposit is provided by performing deposition at a rate higher than 16 .ANG./sec, typically higher than 50 .ANG./sec, and after the deposition, the film is held at a temperature of around 100.degree. C. for several hours to promote the conversion of indium to an oxide form. The indium oxide layer produced by the above-mentioned method was produced at an extremely low oxidation level and the transparency was very poor.
In U.S. Pat. No. 4,345,000 a transparent electrically conductive film and processes for the production thereof are described, the film comprising a metallic oxide wherein the metallic component comprises from 60 to 95 wt % indium and from 40 to 5 wt % tin formed on the surface of a plastic base, wherein the weight ratio of the tin to the indium changes continuously from the outer surface to the inner surface in such a manner that the ratio on the inner surface of the film is appreciably lower than the average ratio of the film and the ratio on the outer surface of the film is appreciably higher than the average ratio of the film. This invention provides an electroconductive film with a high transmittance by such a specific structure.
Cathode sputtering has also been used to form oxide films of the aforesaid metals, particularly indium oxide films. Some of these previous methods of producing sputtered indium oxide films rely on the oxidation of the cathode and the sputtering of this oxide to produce the film. The rate of sputtering is limited by the rate of oxidation of the cathode.
Another method of obtaining indium oxide films is described in U.S. Pat. No. 2,825,687 to Preston. In this patent, indium or another metal oxide is sputtered or the metal is sputtered in an atmosphere containing only a trace of oxygen to produce a colored or opaque film, and the film is subsequently heated in air to develop a permanent conductivity and improve film transmission. The indium oxide-based transparent electroconductive layer obtained this method was also a completely crystallized layer.
U.S. Pat. No. 4,010,291 discloses a method of making low resistance indium oxide conductive films on a substrate by a vacuum evaporation or sputtering process using indium oxide or metallic indium as a starting material in a chamber having an atmosphere comprising an aqueous vapor or gas mixed with an aqueous vapor.
Low resistance indium oxide conductive films made by the invention have the feature that the transmission factor is lowered greatly in the near infrared range so that it is possible to produce infrared selective indium oxide reflecting films of superior performance with good reproducibility; however, this does not disclose any suggestion to produce a highly durable and stable film of the present invention.
Further, U.S. Pat. Nos. 4,065,600, 4,172,020, 4,336,119, 4,379,040, 4,400,254, 4,512,864 and 4,661,229 disclose production methods for an indium oxide-based transparent electroconductive layer by sputtering; however, none suggests the present invention.
The present invention was accomplished by finding and resolving a practical and great problem whereby a transparent electroconductive laminate prepared by depositing an indium-tin oxide layer by sputtering often suffered from the occurrence of disconnection in the sticking process of the transparent electroconductive film to an emitting layer by heat-compression when used as a transparent electrode of an electroluminescent panel.